Arduino is an easy-to-learn prototyping platform that lets you create interactive electronic projects. This Arduino compatible kit is the one I recommend to people who ask me how to get started. The reason I recommend it is because it's very cheap and it has a bunch of components that would cost a lot more if you were to purchase them separately.
Two things it doesn't have, but should if you really want to have fun with Arduino: a potentiometer and a servo motor. This kit, which has these components and many more, is available for about twice as much as the basic kit above.